Method and apparatus for the quantitative, depth differential analysis of solid samples with the use of two ion beams

ABSTRACT

A method and an apparatus for the quantitative depth analysis of a solid sample by backscatter analyzing the sample with the light ions, removing a thin layer of the sample by sputter etching, using a beam of medium-mass or high-mass ions to bombard the sample, backscatter analyzing the sputter etched sample, and repeatedly performing the steps of removing a thin layer of the sample and backscatter analyzing the sputter etched sample. An apparatus for performing the method includes an analysis chamber for retaining the sample to be analyzed, and first and second accelerators. The first accelerator generates fast, light ions with an energy from about 0.1 MeV to about 5.0 MeV to be directed into the chamber onto a predetermined region of the sample at a first desired predetermined bombardment angle so that the fast ions are scattered by the ions of the sample. The second accelerator accelerates a beam of slow medium-mass or high-mass ions with an energy from about 0.5 to about 10.0 keV to be directed onto the predetermined region of the sample at a second desired predetermined bombardment angle. The analysis chamber has a sample manipulator for manipulating the sample and an analyzer for determining the energy of the fast ion scattered by the atoms of the sample.

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for quantitative, depthdifferential analysis of solid samples with the use of two ion beams,namely by Rutherford backscattering of light ions and by sputter erosiondue to bombardment with medium-mass or high-mass ions. The invention isbased on the idea that the analytical potential of the Rutherfordbackscattering technique can be broadened and increased substantially ifthis method is employed in combination with sputter sectioning of thesample.

The principle and uses of backscattering spectrometry are described indetail in the book by Wei-Kan Chu, James W. Mayer and Marc-A. Nicolet,entitled Backscattering Spectrometry, published by Academic Press, NewYork, 1978. The nomenclature employed in this book will also beessentially employed in the description of the invention given below.References to relevant equations or paragraphs of the book will bepreceded by "Chu et al".

The backscattering spectrometry method employs a beam of fast, lightions (i.e. He⁺ or He²⁺) which is directed onto a sample. The desiredinformation about the composition of the sample is obtained by measuringthe energy spectrum of the primary particles which are scattered intothe solid angle ΔΩ around angle θ. If an atom of mass M₂ is located at adepth z of the sample, with z being measured perpendicularly to thesurface of the sample, a primary particle of mass M₁ and an initialenergy E₀, after being scattered off M₂, exhibits the energy E₁ whenleaving the sample. This energy can be expressed as follows (Chu et al,§§3.2.1 and 3.2.2):

    E.sub.1 =KE.sub.0 -[ε]Nz                           (1)

with the stopping cross section factor [ε] being given by

    [ε]=Kε.sub.in /cosθ.sub.in)=(ε.sub.out /cosθ.sub.out)                                      (2)

where K is the so-called kinematic factor which, for a given scatteringangle θ (the angle between the directions of the incoming particle, andthe exiting particle after scattering), depends only on the mass ratioM₂ /M₁ (Chu et al, §2.2 as well as Tables II to V). The value εindicates the mean stopping cross section of the sample for the primaryparticle along its path between the surface and the scattering center.Subscripts `in` and `out` designate the incoming and exiting particle,respectively. The symbols θ_(in) and θ_(out) signify the angles betweenthe surface normal and the propagation directions of the incoming andexiting particle bundle. N is the density of the sample in atoms/cm³(See Chu et al. §§3.2.l and 3.2.2).

Without other knowledge about the composition of the sample beingexamined, Equation (1) cannot be solved since it is--even with theknowledge of ε--an equation with two unknowns, namely K and z.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to modify theabove-described method in such a way that the equivocality of thebackscattering spectrum is eliminated.

According to the invention this problem is solved in steps. Initially aspectrum is recorded of the untreated sample (hereafter called thevirgin spectrum). Then a beam of slow, heavy ions (e.g. Ar⁺) is employedto remove a layer thickness element Δz_(j) from the sample bysputtering, whereupon a further backscattering spectrum is recorded(spectrum j). Because of the removal of the layer of thickness Δz_(j),the characteristic structures observed in the virgin spectrum areshifted in spectrum j, according to Equation (1), toward higher energiesby the following amount:

    ΔE.sub.1,j =[ε]NΔz.sub.j               ( 3)

By measuring ΔE₁,j and Δz_(j) it is now possible, according to Equation(2) and with the knowledge of θ_(in) and θ_(out), to unequivocallydetermine K and thus M₂ if the energy dependent stopping cross sectionε=ε(E) is known from literature values (e.g. from Chu et al, Table VI,pages 362, 363). This situation exists if a sample having unknownimpurities but known major components.

If ε(E) is unknown, then the removal of the sample material bysputtering must continue until the atoms of mass M₂ *, which produce acharacteristic structure in the back-scattered spectrum, reach theinstantaneous surface. In this case, the instantaneous depth z' definedby the following equation ##EQU1## has become zero so that K(M₂ *) isunequivocally determined according to Equation (1), i.e.

    K(M.sub.2 *)=E.sub.1 (z'=0)/E.sub.0                        ( 5)

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1e are respective schematic cross-sectional views of a sampleto be analyzed at five successive stages of sputtering;

FIGS. 1a'-1e' are graphs of the energy spectrum of the sample at therespective stages shown in FIGS. 1a-1e;

FIG. 2 is a schematic illustration of a backscattering analysis of asputtered sample at a glancing angle of incidence; and

FIG. 3 schematically illustrates a structural arrangement according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will now be described in greater detail with reference toa simple example and FIGS. 1 to 3.

Let it be assumed that the sample is a multi-layer sample having thestructure shown in FIG. 1a. A thick layer 18 of polycrystalline silicon(poly-Si) is deposited on a substrate, which is not of interest here,and is covered with a layer 19 of silicon dioxide (SiO₂). A narrowregion 20 of the poly-Si layer 18 is doped with arsenic.

The backscattering spectrum of such a sample is shown schematically inFIG. 1a'. The vertical arrows identify, according to Equation (5), therelative energy E₁ (z'=0)/E₀ for elements ¹⁶ O, ²⁸ Si, ⁷⁵ AS and ¹³¹ Xe.If now--as shown in FIGS. 1b to 1e--thin layers are sputtered away instages, for example by bombarding the sample with xenon ions of anenergy of a few keV, the backscattering spectra will change in themanner shown in FIGS. 1b' to 1e'. Using Equations (5) and (1), it ispossible to identify, from the position of the characteristic stages andmaxima and possibly their shift during continued sputtering, theelements contained in the sample and their position and depth in thesample.

The approach according to the invention is primarily of great utilityif, as in the example of FIG. 1, impurity or doping elements of massM₂,i are present at a depth z>z_(i),m in a matrix of lighter atoms whichhave a mass M₂,m, where

    z.sub.i,m =(K.sub.i -K.sub.m)E.sub.0 /N[ε]         (6)

where K_(i) and K_(m) are, respectively, the kinematic factors for theimpurity element of mass M₂,i and the lighter atoms of mass M₂,m.

In this case, the signals generated by the two types of atoms (theimpurity and the lighter atoms) are superposed in the backscatteringspectrum at energy E₁ in FIG. 1a' the arsenic signal shows up on a"background" due to silicon in SiO₂). The "background", which is veryannoying for a quantitative determination of the concentration of M₂,iatoms, can be eliminated if a layer of a thickness Δz is removed fromthe sample by sputtering so that z-Δz<z_(i),m (see migration of thearsenic signal in FIGS. 1a' to 1c').

A second advantage of the method according to the invention is that acombination of the backscattering method with bombardment inducedsputtering permits the analysis also of deep lying regions in thesample. Without sputter sectioning, the maximum depth z_(max) detectableduring backscattering can be estimated with the aid of Equation (1) bysetting the backscattering energy E₁ to zero, i.e.

    z.sub.max =KE.sub.0 /N[ε]                          (7)

The limitation defined by Equation (7) can be overcome according to theinvention if, between two successive backscattering analyses, a layer ofsuitable thickness Δz_(j) is removed from the sample. In order todetermine the composition of the sample continuously as a function ofthe depth, the thickness Δz_(j) should be selected in such a way thatthe backscatter spectra determined before and after atomization overlapin part, i.e.

    Δz.sub.j =βz.sub.max                            (8)

with

    0.2<β≦0.7

A third advantage of the method according to the invention relates tothe possibility of performing backscattering analyses (see Chu et al,§§7.4, 7.5) with great depth resolution not only in the vicinity of thesurface but also at a greater depth. According to Equation (1), thedepth resolution δz can be represented as follows

    |δz|=δE.sub.1 /N[ε]  (9)

It follows from Equation (9) that δz becomes smaller, i.e. the depthresolution becomes better, the smaller is δE₁ and the larger is [ε]. Theenergy width δE₁ is composed of two parts, the given energy resolutionδE_(r) of the backscattering arrangement and the energy stragglingδE_(s) (Chu et al, §7.4) which, in the Gaussian approximation, is givenby:

    δE.sub.1 ={(δE.sub.r).sup.2 +(δE.sub.s).sup.2 }.sup.1/2(10)

Energy straggling increases in proportion to the square root of the pathlength traversed by the beam in the sample. A good energy resolutionδE_(r) can thus be utilized fully only if δE_(s) <δE_(r), i.e. the depthrange that can be analyzed with a narrow energy width δE₁ is limited.This applies particularly if, for the purpose of high depth resolutionδz, the backscattering measurement is performed at a glancing angle ofincidence and/or exit of the analyzing beam so that [ε]becomes large(see Equations 2 and 9 as well as Chu et al, §7.5). The region coveredwith high depth resolution then becomes very narrow. According to thepresent invention one can analyze the sample with high depth resolutionalso at a greater depth if sample material is removed in steps by way ofsputtering. In this case it is useful to perform the sputtering withheavy ions (e.g. Xe⁺) of an energy of less than 1 keV and by bombardingat a glancing angle of incidence (θ_(sputtering) >60°). In this way theregion 21, which was radiation damaged during sputtering, will extendonly to a small depth from the instantaneous surface (see FIG. 2).

An arrangement for implementing the method according to the invention isshown in FIG. 3. The beam of fast, light ions 1 is produced by aconventional accelerator 1a with associated analysis magnet (not shownin detail here). On its path along the evacuated beam tube 2, the beamis collimated by means of apertures 3 to 6 before it impinges on asample 7 (which is shown in detail in FIG. 1a) on a conventional samplemanipulator 7' in analysis chamber 22 as schematically illustrated inFIG. 3. Sample 7 can be rotated about the v axis perpendicular to theplane of the drawing and can be moved in the directions of the u, v andw axes, by manipulator 7'. The primary particles 17 which arebackscattered from atoms of sample 7 generate a signal in detector 8which is processed in an electronic data acquisition system 8'schematically illustrated in FIG. 3.

The ion beam 9 used for the sputtering is generated by means of a smallaccelerator 10. In order to filter its velocity (or its mass, at a givenbeam energy) beam 9 passes through a Wien filter 11 and an aperture 12.Beam 9 is focused on sample 7 by means of an Einzel lens 13. By applyingsuitable, time-dependent voltages across a pair of double plates 14, thefocused beam can be master scanned in a TV pattern over sample 7 so thatan area of a magnitude A_(s) is bombarded with a constant currentdensity when averaged over time. This ensures uniform removal ofmaterial from sample 7 (having portions 18-20) over area A_(s). To beable to realize a high removal rate with a given current of ion beam 9,area A_(s) should be equal to or only slightly larger than the areaA_(r) covered by the analyzing beam 1.

For analyses of bombardment at a glancing angle of incidence, i.e. at80°<θ_(in) <90°, it is appropriate to give one or several of apertures 4to 6 a slit shape so that the slit width b_(u) is much less than theslit height b_(v). With a sufficiently small divergence of beam 1, it ispossible, in the case of b_(u) =b_(v) cos θ_(in), to give A_(r) a squareshape with edges of a length b_(v).

Absolute measurements of the ion currents incident on sample 7 can beperformed by means of Faraday cups 15 and 16, respectively. For acurrent measurement, the sample must not intercept the respective beam.This can be accomplished by moving the sample in one of directions u, vand w.

No limitations exist with respect to the arrangement of the axes ofbeams 1 and 9 relative to one another. It must merely be ensured thatsample 7 can be irradiated by both beams 1 and 9 at freely selectablebombardment angles.

The present disclosure relates to the subject matter disclosed in theFederal Republic of Germany, P No. 38 03 424.7 on Feb. 5th, 1988, theentire specification of which is incorporated herein by reference.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

What is claimed is:
 1. A method for the quantitative depth differentialanalysis of a solid sample, comprising the steps of:(a) backscatteranalyzing the sample with light ions; (b) removing a thin layer of thesample by sputter etching, using a beam of medium-mass or high-mass ionsto bombard the sample; (c) backscatter analyzing the sputter etchedsample; and (d) repeatedly performing said steps (b) and (c).
 2. Amethod as in claim 1, wherein each backscatter analyzing step includes astep of directing said light ions onto the sample at a glancingincidence and each said step of removing includes the step of directingthe beam of medium-mass or high-mass ions onto the sample at low ionenergy and so as to have a glancing incidence on the sample, wherebyboth shallow and deep lying regions of the sample are exposable foranalysis with high depth resolution.
 3. A method as in claim 1, whereineach backscatter analyzing step includes a step of detecting thebackscattered light ions at a glancing angle of emergence, and each stepof removing by sputter etching includes the step of directing the beamof medium-mass or high-mass ions onto the sample at a low ion energy andso as to have a glancing incidence on the sample, whereby both shallowand deep lying regions of the sample are exposable for analysis withhigh depth resolution.
 4. A method as in claim 10, wherein eachbackscatter analyzing step includes a step of directing said light ionsonto the sample at a glancing incidence.
 5. A method as in claim 1,wherein each backscatter analyzing step includes a step of directingsaid light ions onto the sample at a glancing incidence.
 6. An apparatusfor the quantitative depth differential analysis of a solid sample,comprising:analysis chamber for holding therein the sample to beanalyzed; first accelerator means for generation a beam of fast, lightions with an energy in a range from about 0.1 MeV to about 5 MeV anddirecting the beam of fast ions into said chamber onto a predeterminedregion of the sample at a desired bombardment angle so that the fastions are scattered by the atoms of the sample in the predeterminedregion thereof; and second accelerator means for generating a beam ofslow, medium-mass or high-mass ions with an energy of about 0.5 to about10 keV and directing the beam of slow ions onto the predetermined regionof the sample at a second desired predetermined bombardment angle, saidanalysis chamber having a sample manipulator means for manipulating thesample and means for determining the energy of the fast ions scatteredby the atoms of the sample.
 7. An apparatus as in claim 6, wherein thefast ions are selected from the group of ions consisting of H⁺, He⁺,He²⁺ and Li⁺.
 8. An apparatus as in claim 7, wherein the slow ions areinert gas.
 9. An apparatus as in claim 7, wherein the slow ions compriseAr⁺.
 10. An apparatus as in claim 7, wherein the slow ions comprise Xe⁺.11. An apparatus as in claim 6, wherein the slow ions are inert gas. 12.An apparatus as in claim 6, wherein the slow ions comprise Ar⁺.
 13. Anapparatus as in claim 6, wherein the slow ions comprise Xe⁺.